1. Field of Invention
The invention relates to a stage system, a lithographic apparatus including such stage system, and to a correction method to correct a position measurement system in a dual stage area lithographic apparatus.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
To be able to measure a position of the substrate table (also referred to as substrate stage), a measurement system may be provided in the lithographic apparatus. Likewise, a position measurement system may be provided to measure a position of the mask table or patterning device support (also referred to as mask stage) which is arranged to hold the patterning device or mask. For such position measurement systems, use may be made of many measurement principles, such as interferometer(s) encoder(s) etc. In such measurement systems, use may be made of reference plates, such as encoder plates. Thereby, a sensor of the measurement system may be connected to the stage, thus moving with the stage and following the position of the stage, while the sensor being arranged to perform a measurement from which a position or displacement or distance information of the sensor relative to the reference plate, may be determined. In such solutions, a plurality of sensors may be provided on the stage, e.g. at various edges, sides etc. thereof. Depending on a position of the stage relative to the reference plate or reference plates, a subset of the sensors may be active, as in some of the positions one or more of the sensors may be outside a reach of the plate in question, or may be in an area between adjacent plates, etc. Therefore, depending on a position of the stage relative to the plates, different subsets of the sensors may be used to determine the position of the stage. Also, due to many causes of disturbance, a position measurement by the sensors may be inconsistent. As an example, a position of such sensor relative to the stage may be subject to a tolerance, temperature drift, etc., a position of one or more of the plates may be subject to drift or other disturbances, the plates themselves may be subject to fluctuations such as thermal expansion, etc.
By such disturbances, inconsistencies in readout of the sensors may occur. Due to such inconsistencies, several phenomena may be observed. As an example, assuming that a stage with 4 sensors is provided, while at a first position sensors 1-3 are applied to measure the position of the stage, while at a second position, sensors 1, 2 and 4 are applied thereto, while a drift of sensor 4 has occurred. Then, a position determination of the first position from sensors 1-3 may be inconsistent with a position determination from sensors 1, 2 and 4, causing a control system, such as a feedback control system to control a position of the stage, to correct the position of the stage based on the position determined from (a subset of) the sensors, hence causing the stage in the second position to be corrected by such feed back operation differently which may result in an offset, rotation, etc., in other words a mismatch of the stage position at the second position relative to the stage position at the first position. As a consequence thereof, offset errors, alignment errors, overlay errors etc., may occur.